This invention relates generally to electrical control devices for opening an electrical circuit in response to an abnormal overcurrent condition, and, in particular, to such devices that delay opening the electrical circuit for a time period which is inversely related to the magnitude of the overcurrent condition.
Various known protective devices exist for opening an electrical circuit in the event of an abnormal fault or overcurrent condition, i.e., a condition in which current through a conductor exceeds a predetermined maximum threshold value. In electrical power distribution systems, momentary overcurrents of small to moderate magnitude are frequent and can result, for example, from a tree branch swaying into and out of contact with a power line. Because small to moderate overcurrents of short duration pose no immediate threat to the well-being of the electrical power distribution system, it is desirable that protective devices used in such distribution systems exhibit inverse time-current response characteristics, i.e., provide delayed response to overcurrents of small magnitude and rapid response to overcurrents of large magnitude. The use of protective devices exhibiting such response characteristics avoids the inconvenience of service interruptions resulting from momentary overcurrents which pose no immediate threat to the system, while assuring that large overcurrents will be interrupted before damage to the power distribution system can occur.
Known electro-mechanical protective devices, such as fuses, circuit reclosers and circuit breakers, respond to the physical heating of a conductor and, thus, inherently provide the desired inverse time-current response characteristic. However, such electro-mechanical protective devices as circuit breakers and circuit reclosers are subject to the wear and maintenance problems which plague all mechanical devices. Fuses, which cannot be reset and which must be replaced when blown, can give rise to inconvenience and expense. Although purely electronic systems for sensing overcurrent conditions avoid the practical difficulties associated with electro-mechanical devices, such electronic systems do not inherently provide the desired inverse time-current response characteristic--such a characteristic must be designed into the system.
To minimize the extent of a service interruption in the event of a persisting overcurrent condition, it is a known practice to install protective devices so that devices having relatively faster response characteristics are positioned farther downstream from the power station than are devices having relatively slower response characteristics. This helps assure that only the protective device nearest a fault opens and thus helps avoid terminating service to branches unaffected by the fault. Because electro-mechanical devices continue to find service in power distribution systems, particularly at the higher current levels, it is desirable that electronic systems for sensing overcurrent conditions be capable of approximating the inverse time-current response characteristics of such electro-mechanical devices and that some provision be made for tailoring the response of the electronic system so as to obtain a response which is appropriate in view of the intended location of the electronic system within the power distribution system.
One electronic system for providing an inverse time-current response is shown in U.S. Pat. No. 3,105,920 which issued Sept. 15, 1961 to Dewey and discloses a protective device wherein the rate at which a capacitor charges is varied so as to provide the desired inverse time-current response.
Another such system is shown in U.S. Pat. No. 3,944,890 which issued Mar. 16, 1976 to Little on a static overcurrent relay.
Still another such system is shown in U.S. Pat. No. 4,571,658 which issued Feb. 18, 1986 to Ruta on a control circuit for a circuit interrupter.
Attention is also directed to the following U.S. Patents:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 3,976,919 Vandevier et al. August 24, 1976 4,315,296 Hancock February 9, 1982 3,397,323 Hirsch August 13, 1968 3,970,899 Davis July 20, 1976 4,386,384 Moran May 31, 1983 3,808,466 Campbell April 30, 1974 3,840,813 Allen et al. October 8, 1974 3,530,365 Peugh September 22, 1970 3,760,255 Grodinsky September 18, 1973 4,647,861 Petritis et al. March 3, 1987 ______________________________________